1. Field of the Invention
The present invention relates to a laser irradiation method and a laser irradiation apparatus which are suitable, for example, for crystallizing an amorphous semiconductor film and which can project a linear laser beam having homogeneous intensity distribution to an irradiation surface. More specifically, the present invention relates to a laser irradiation method and apparatus using a cylindrical lens which can form a laser beam having homogeneous intensity distribution and which can project the linear laser beam having homogeneous intensity distribution onto an irradiation surface without causing the linear laser beam to have bias in its intensity distribution in reflection of intensity distribution of an original beam.
2. Description of the Related Art
Conventionally, a semiconductor device has been manufactured by crystallizing an amorphous silicon film generally through heat treatment or laser annealing. Since the heat treatment is performed at high temperature, a glass substrate has a problem of being softened due to the heat, for example. The laser annealing is superior to the heat treatment in that such a problem can be avoided. For example, a poly-crystalline silicon film can be obtained when laser irradiation is performed in such a way that after a pulsed excimer laser beam is shaped into a linear laser beam spot by an optical system, the linear laser beam spot is scanned on the amorphous silicon film.
Despite the advantage mentioned above, the laser annealing has a problem of irradiation unevenness on a film because the intensity distribution of the linear laser beam fluctuates when the laser beam is scanned. This irradiation unevenness varies a semiconductor characteristic within the substrate. Therefore, when a display device with a driver and a pixel integrated (system-on-panel) is manufactured with the film having irradiation unevenness, the irradiation unevenness appears directly on the screen of the display device. The irradiation unevenness causes a problem particularly when a display device with high accuracy and high characteristic, typically an EL display, is manufactured.
In the case of forming a linear laser beam by the laser irradiation apparatus, a laser beam emitted from a laser oscillator (hereinafter referred to as an original beam) is generally divided by a cylindrical lens array. Then, the divided laser beams are superposed by a cylindrical lens, and thus a linear laser beam is formed on the irradiation surface. When this linear laser beam is used to process the semiconductor film, it is required that the linear laser beam has high homogeneity in which the energy density fluctuates within several %.
However, when the requirement is not satisfied, for example when the linear laser beam does not have homogeneous energy distribution in the major-axis direction (or longitudinal direction), it is difficult to perform laser processing all over the region in the major-axis direction within the optimum energy range. As a result, it becomes impossible to process the entire surface of the substrate homogeneously, and thus the irradiation unevenness occurs. That is to say, in the case where the energy distribution of the linear laser beam fluctuates in the major-axis direction depending on the intensity distribution of the original beam, the irradiation unevenness occurs in reflection of the change of the intensity distribution of the original beam per unit time.
Generally, in order to increase the homogeneity of the linear laser beam, the number of cylindrical lenses constituting the cylindrical lens array is increased so that the original beam is divided into more beams. However, with the increase of the number of divided beams, the interference between the divided beams become more remarkable, which may result in the appearance of the interference fringes on the irradiation surface. Moreover, when a cylindrical lens array is manufactured with cylindrical lenses each having large aspect ratio between its width and length, enough strength and lens accuracy are difficult to achieve; therefore, the desired beam is difficult to obtain. For this reason, actually, it is difficult to increase the number of divided beams more than the predetermined number in the laser irradiation.
When the intensity distribution of the original beam has bias in one direction, each of the divided beams also has the bias in its intensity distribution. Therefore, the linear laser beam formed by combining the divided beams on the irradiation surface also reflects the bias in the distribution of the original beam. Although the number of divided beams is increased, the linear laser beam has the intensity distribution with the bias in the major-axis direction in the same manner; therefore homogeneous processing is impossible. As thus described, in the case of homogenizing the laser beam with the conventional cylindrical lens array, it has been impossible to form the homogenous laser beam on the irradiation surface when the original beam does not have symmetrical intensity distribution.
Furthermore, in the case of a gas laser, it is necessary to cool a tube with medium gas enclosed in which a laser beam is excited. Since the oscillation characteristic of the laser is affected by, for example, the fluctuation of the cooling water flow rate, the intensity distribution of the original beam may fluctuate. Moreover, it has been known that the intensity distribution of the original beam may fluctuate due to thermal distortion of an element in the resonator. In addition, the intensity distribution of the original beam may fluctuate due to various other factors. An optical system including the conventional cylindrical lens is sensitive to the fluctuation of the original beam.
In order to process the object homogeneously and stably, an optical system is required which can form a homogeneous linear laser beam constantly without depending on the intensity distribution of the original beam. Moreover, an optical system is required which is less sensitive to the fluctuation of the intensity distribution of the original beam. For these reasons, a laser irradiation method and a laser irradiation apparatus have been required which can project the linear laser beam having homogeneous intensity distribution without being affected by the intensity distribution of the original beam. A laser annealing apparatus including a cylindrical lens has been already applied for a patent by the present inventor (see Reference 1: Japanese Patent Application Laid-Open No. 10-253916). This laser annealing apparatus includes a cylindrical lens array having convex cylindrical lenses and concave cylindrical lenses arranged alternately.